Preheat the grill to high temperature. In a medium sauce pot, bring the ale to a boil and reduce by 3/4. Add the lemon and remove the pot from heat. Whisk in the butter until it is melted. Add the horseradish and season with salt and pepper. Keep warm. In a sauté pan set over medium-high heat, heat the olive oil for two minutes. Add the shallots and sauté until translucent, about one minute. Add the spinach and sauté for one minute more. Keep warm. Place the oysters on the grill and cook until they begin to open—about six to eight minutes. Remove the oysters from their shells, reserving the bottom half of the shells. Set a pinch of spinach into each shell, top with an oyster and drizzle with the ale butter. Serve immediately.